Conventional sheet-making machinery for producing a continuous web or sheet of material includes equipment to set the sheet properties of the web as it is being manufactured. Generally, on-line measurements of sheet properties are made by scanning sensors that travel back and forth across the width of the sheet of material in the cross-machine direction (CD). The machine direction (MD) is the direction of travel of sheet. The scanning sensors are located downstream of actuators that are controlled to adjust the sheet properties. The scanning sensors collect information about the sheet properties to develop a property profile across the sheet and provide control signals to the appropriate actuators to adjust the profile toward a desired target profile in a feedback loop. In practice, the actuators provide generally independent adjustment at adjacent cross-directional locations of the sheet, normally referred to as slices.
In paper-making equipment, properties such as paper weight, thickness (caliper), smoothness, moisture content, and gloss are controlled by manipulating appropriate actuators to adjust the properties under the actuators' influence toward a desired goal.
High-performance cross-directional (CD) control of sheet-making machines, particularly, paper machines, requires accurate knowledge of the controlled process model. Particularly important for CD control is an accurate knowledge of the mapping between CD actuators and their response centre positions in the measurement scan. Mapping involves establishing the relationship between each downstream slice where scanning measurements occur and the corresponding upstream actuator that must be adjusted to control the particular downstream slice. In practice, this mapping depends on the paper alignment and shrinkage, which vary from one paper machine to another and with time for the same machine.
Even though mapping is used, conventional control systems still rely on actuators that have a fixed footprint that affects a particular slice of the sheet under manufacture. Only the magnitude of the actuator response is manipulated by conventional control systems to adjust sheet properties and there is no attempt to dynamically manipulate response shape. In some cases, the actuator footprint shape may change but this change is not controlled directly and is a consequence of manipulation of the magnitude of the actuator response.